KR101591712B1 - Binders in anode materials of lithium secondary battery - Google Patents

Abstract

The present invention reduces the expansion of each of the silicon, silicon-carbon composite, and silicon alloy-carbon electrode active material, which is a high-capacity anode material, and is excellent in bonding strength between the active material particles and the collector, And more particularly to a binder for a lithium secondary battery negative electrode suitable for each active material to be excellent.
The binder for a lithium secondary battery negative electrode according to the present invention is a binder for an anode active material of a composite secondary battery to which 20 wt% of carbon is added to 80 wt% of silicon, and carboxymethyl cellulose is mixed with polyacrylic acid having a low water molecular weight of 450,000 .

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a binder for lithium secondary batteries,

The present invention relates to a negative electrode material having a high capacity of a lithium secondary battery and excellent in adhesion and dispersibility when a silicon (Si), a silicon-carbon (Si-C) composite and a silicon alloy- (PAALi) in which a hydroxyl group (OH), which is a functional group of a water-based polyacrylic acid (PAA), is substituted with lithium (LiAl), and a binder having a high capacity cycle life and an expansion ratio. (CMC) as a modifier for improving the adhesion property to polyacrylic acid having a molecular weight, hydroxypropylcellulose (HPC) capable of acting as a dispersant in a polyamideimide (PAI) or polyamideimide, ), A slurry for an electrode prepared using the binder, an electrode made using the slurry, and a lithium secondary battery negative electrode made of the electrode It relates to a binder.

Recently, lithium-ion secondary batteries are being developed for use in electric vehicles and energy storage of renewable energy, and they are being developed into medium- and large-sized batteries with a high capacity, high output, long life, and high stability in a small-sized center of a conventional home. Such a middle- or large-sized lithium ion secondary battery should have a high energy density, be thin and short, and be low cost so that it can be commercialized.

However, in the conventional small-sized lithium ion secondary battery, lithium cobalt oxide and lithium manganese oxide were used as an anode and a graphite system was used as an anode. In the case of a carbon anode material, since the conductivity of carbon is excellent and a small amount of binder is used, the conductivity of the binder is greatly influenced by the binder used to form the electrode material into a slurry or a sheet. I did not. In general, polyvinylidene fluoride (PVDF) binders having excellent bonding strength with stadiene-butadiene rubber (SBR) have been mainly used for carbon because they have almost no swelling.

In the case of using the current binder polyvinylidene fluoride and stadiene-butadiene rubber such as silicone, silicon-carbon composite and silicon alloy-carbon composite, which are high-capacity cathode materials, an excessive amount of binder should be used for maintaining the binding power. The electric conductivity is extremely lowered, and the cycle life is significantly lowered. In addition, it expands due to reaction with most organic electrolytes such as propylene carbonate and dimethoxyethane. In other words, the polyvinylidene fluoride as a binder of the binder has a weak binding force due to the linear bonding and is expanded and expanded so that the negative electrode particles have a sufficient space for cracking, and the aqueous binder, stadiene-butadiene rubber, But has a relatively weak binding force.

In addition, silicon has a capacity (theoretical capacity of about 4200 mAh / g) more than 10 times that of graphite, which is the main anode material. However, when lithium ions intercalate with a silicon anode material, Therefore, when the battery is repeatedly charged and discharged, the volume change of 300% or more occurs due to the lithium ion intercalation. However, since the binder used for the negative electrode has the function of crosslinking the particles of the negative electrode active material and the function of adhering to the collector, if the binder is not suitable for the negative electrode material, the structure is broken, peeled or deformed, The volume expansion of the negative electrode active material may be greatly reduced by using the elasticity of the binder and the point binding property.

Therefore, in order to manufacture a negative electrode capable of commercialization of an electric vehicle or a mid-sized lithium ion secondary battery for electric power storage, the development of a binder must be made indispensable. The binder is influenced by charge and discharge profile, swelling and lithium migration resistance, and intensity and location of C-V peak depending on binder type, amount, molecular weight, crosslinking density and solvent. In addition, it affects peeling, peeling, and deformation of the negative electrode sheet in the current collector, so it needs to be developed in consideration of this.

As the high-capacity anode materials being developed at present, there are silicon and lithium having a carbon content of 5 times or more and silicon, lithium alloy, lithium alloy, lithium nitride-transition metal, silicon oxide, amorphous tin composite oxide and transition metal vanadium oxide However, to date, some binders have been reported by some researchers as [1-3].

[Document 1]

Gao Liu (Advanced Materials 23, 4679 (2011)) discloses a polyvinylidene fluoride (PVdF), a polyacrylonitrile (PAN), and a polydioctylfluorene cofluorene comonomethylbenzoic ester PFFOMB) binder to charge and discharge up to 14 cycles to maintain a capacity of 2520 mAh / g.

[Document 2]

NSChoi (J Power Source, 187, 590 (2008)) compared polyvinylidene fluoride (PVDF) with polyamideimide (PAI) binder and found polyvinylidene fluoride has an initial efficiency of 28.3%, expansion 221 % Or polyamide imide was better at 74.9% initial efficiency and 160% expansion. However, as mentioned above, the polyamide imide having excellent properties for the silicone system is required to be subjected to heat treatment for a long time in a high-temperature vacuum atmosphere of 300 or more after coating with the current collector, so that the productivity is greatly lowered and EH is environmentally harmful There is a problem.

[Document 3]

Alexander Magasinski (ACS Appl. Mater. 2 (2010) 3004) uses a polyacrylic acid (PAA) binder for the silicon nano particle anode material. The ratio of silicon nanoparticles: Carbon black: binder is 43 : 42: 15. As a result, it was found that the binder ratio of polyacrylic acid (PAA) was 2000 mAh / g (coulombic efficiency 70%) at 100 cycles and the ratio of carbon coated carbon: The cycle life was high at 2500 mAh / g (75% coulombic efficiency) at 17: 43: 25: 15. When carboxymethylcellulose and polyvinylidene fluoride were used as binders, they were 800 mAh / g and 100 mAh / g. < / RTI >

In these studies, silicon particles have to be nano-sized, and when using expensive conductive materials, the productivity is reduced, the cycle is too short, the expansion is too large, or the amount of the conductive material is too large to be commercialized. Nonetheless, no suitable binder for silicon-carbon composites and silicon alloy-carbon composites using low cost natural carbon as well as silicon alone has been reported.

It is an object of the present invention to provide a high-capacity active material which, when mixed with 90 weight percent silicon nanoparticles and black carbon as a 10 weight percent conductive material, combines 10 to 20 weight percent silicon nanoparticles and 90 to 80 weight percent carbon particles with an active material When the silicon alloy active material is 45 to 92 weight percent, the active alloy composition should be used when the silicon content is 66 to 69 weight percent and the balance Ni, Fe and Ti is 8 to 45 weight percent carbon The present inventors have invented a binder having the best capacity, initial efficiency, and cycle life of a lithium ion secondary battery cathode, which is the most suitable binder among various kinds of binders, that is, a binder excellent in dispersion and bonding of active material particles and binding between a current collector and an electrode sheet.

The present invention relates to an anode material having a high capacity of a lithium secondary battery, and a method of using the silicon material, the silicon-carbon composite material, and the silicon alloy-carbon composite material, in order to maintain the capacity even after repeating a high initial capacity and cycle, 2, 3, and 7 by selecting a binder which can strongly bond the active material particles to the particles and have a strong adhesive force with the current collector while reducing the expansion, , 12 < Tables 1, 2, 3, 4, >, lithium and silicon reacted during the cycle to reduce the expansion. And further to disperse the active material particles and the conductive agent and to disperse the binder.

Examples of the binder of the present invention include a water-based low molecular weight polyacrylic acid (molecular weight: 450,000), a high molecular weight polyacrylic acid (molecular weight: 1,000,000 to 1,250,000, a polymerization degree of 13,000 to 18,000), a high molecular weight polyacrylic lithium substituted with a hydroxyl group of lithium, polycarboxymethyl cellulose, Amide imide, and hydroxylpropyl cellulose which can be used simultaneously in water and oil. The inventors of the present invention have invented a binder having excellent cell performance and reduced expansion when used singly or in combination.

In this case, when the polymer polyacrylic acid is modified to include lithium, it is preferable that the polymeric polyacrylic acid includes 0.1 to 100 parts by weight based on 100 weight percent of polyacrylic acid, and the modified polyacrylic lithium, low molecular weight polyacrylic acid, polyamideimide, , A dispersant, carboxymethyl cellulose, hydroxylpropylcellulose, and the like is preferably 6 to 20 weight percent.

On the other hand, according to the silicon system reported by Xiao Hua Liu (ACS Nano 6 (2012) 1522) in Document 4, cracks are not generated in the entire surface particles only when the particle size is about 150 nm or less. However, if the primary particle size of the particle size is about 150 nanometers, agglomeration may occur and it may be difficult to use the active material by lowering the migration of lithium ions.

Accordingly, in the present invention, a dispersant or binder capable of uniformly dispersing the silicone-based active materials in the solvent and capable of improving the dispersibility of the water-based and oil-based binders, and hydroxypropyl cellulose, In the binder.

At this time, the amount of the dispersing agent to be added is preferably 3 to 7 weight percent throughout the electrode. It is preferable that the binder and the dispersing agent are included in an amount of 6 to 20 weight percent based on the total weight of the electrode material. (Water, NMP) is added after mixing the above-described electrode binder and dispersant with a silicon active material (silicon, silicon-carbon composite, silicon alloy-carbon composite) When the coating is applied to the current collector after the formation of the negative electrode, a side reaction during charging and discharging and an additional electrical reaction (see FIG. 4) for substitution such as lithium (Li) And a lithium ion secondary battery comprising the lithium ion secondary battery negative electrode and the negative electrode.

According to the present invention, it is possible to suppress the cracking of the silicon-based particles when the silicon-based electrode active material and the electrode binder of the present invention are mixed with each other during the charging and discharging of the lithium ion secondary battery and coated on the current collector in a slurry state, The increase in distance due to the volume expansion could be suppressed. In addition, an effect of increasing cycle life characteristics is obtained by electrochemically reacting lithium (Li), which is a decomposed material in the reaction, with a substituent functional group of a binder to form a new lithium alloy phase, thereby reducing oxidation progression as the cycle progresses. In addition, an appropriate adhesion force between the electrode binder of the present invention and the current collector of the electrode is provided to form a stable electrode.

In addition, mixing of a nano-sized silicon electrode active material or a micron-sized silicon alloy system and a conductive agent such as carbon black or nano-sized carbon black or ketch-black or hydroprocessing agent is added with a dispersing agent to uniformly distribute these materials .

For the above reasons, the negative electrode material of the present invention has improved battery design capacity and cycle life characteristics, and also has the effect of lowering the expansion ratio.

Figure 1 is a polyacrylonitrile modified (modified) by the infrared spectroscopic analysis of the synthesized lithium polyacrylate binder (Fourier transform infrared spectroscopy) Curve a of the polyacrylic acid hydroxyl group absorption is reduced and the wave number of 3250cm ^-1 alkali metal combined peak Of the absorbance.
Fig. 2 is a graph of charge / discharge when a binder modified with polyacrylic acid and a comparative binder are used.
3 shows the cycle life characteristics of a binder modified with polyacrylic acid and a comparative binder.
4 is a graph showing changes in the crystalline phases of the binder modified with polyacrylic acid and the comparative binder before and after charging and discharging.
FIG. 5 shows a change in the crystal phase (after 500 cycles) after charging / discharging of the polyacrylic acid binder and charging / discharging of the polyacrylic lithium binder.
6 shows the surface and cross-section of the electrode observed with a scanning electron microscope (SEM) after 500 cycles for the negative electrode prepared using the modified binder polyacrylic lithium.
7 is a comparison of the cycle life characteristics of the anode material silicon-carbon composite by binder.
8 is a thermal analysis (DSC) graph for a low temperature binder for replacing a polyamideimide binder for a high temperature of an anode material silicon alloy-carbon composite.
9 shows the microstructure and crystallinity of the silicon-carbon composite.
Particle size: 5 to 15, pore volume: 0.03 m ² / g, specific surface area: 2.9 m ² / g
10 is a carbon sample used in a silicon-carbon composite, a silicon alloy-carbon composite.
Average particle size: 18, specific surface area: 2.15 m ² / g, crystallinity: single phase
11 shows cycle life characteristics (25 times) according to electrode composition when a binder is used as a polyamideimide in a silicon alloy system.

The present invention will be described in more detail so as to enable those skilled in the art to easily carry out the present invention.

The present invention requires different binders for silicon, silicon-carbon composites and silicon alloy-carbon composites of high capacity cathode materials. The negative electrode material alone exhibited excellent properties in the polyacrylic lithium binder, but exhibited low cycle life characteristics in the silicon-carbon bond composite as shown in Fig. 7, and the polyacrylic lithium binder in the silicon alloy- Respectively. Therefore, we have tried to invent a binder suitable for each of the anode active materials mentioned above.

First, a polyacrylic lithium binder suitable for silicon is to provide a binder for a negative electrode which has a high binding force to silicon or reacts electrochemically to have a high charge / discharge capacity, excellent cycle life (capacity retention rate) and low expansion rate. Particularly, the polyacrylic lithium binder substituted with lithium ion in polyacrylic acid significantly improved the life cycle of the electric cycle by the addition of lithium, which is the main reactant of the electrochemical reaction, and the side reaction with the negative electrode due to the decomposition of organic materials during the use of the electrolyte. Polyacrylic acid having a degree of polymerization of 13,000 to 18,000 (molecular weight: 1,000,000 to 1,250,000) of a polyacrylic lithium binder is modified with polyacrylic lithium. The optimum degree of polymerization of the polyacrylic lithium modified with the polyacrylic acid binder was 15,000 to 16,000 (molecular weight: 1,000,000 to 1,250,000). When the degree of polymerization is less than 13,000, the adhesion to the current collector is insufficient. When the degree of polymerization is more than 18000, the production of the electrode sheet is difficult due to the high viscosity.

As mentioned earlier, suitable binders for silicon were not suitable in the silicon-carbon composites. Although low molecular weight polyacrylic acid was intended to be used as a binder for this system, it was superior in elasticity and binding power and mixed with carboxymethyl cellulose in polyacrylic acid to increase it. The degree of polymerization of low molecular weight polyacrylic acid was 5000 to 8000. And a binder including polyacrylonitrile, carboxymethyl cellulose, polyamide imide and the like, which have been conventionally known, have been examined. In addition, in the case of the silicon alloy-carbon composite, it was further difficult to apply the acrylic lithium binder to the paste. At present, the polyamide-imide, which is a flowable resin, has the most excellent binder characteristics, but it has to be heat-treated for a long time at a high temperature, resulting in a decrease in productivity and a high harmful binder. Therefore, the present invention has been made to overcome this problem. In order to increase the yield of the silicone alloy-based carbon composite, a mixed binder which exhibits performance improvement, that is, a reduction in the expansion ratio, by adding hydroxylpropylcellulose which is excellent in dispersibility and is a binder which can be applied to both oil and water systems at the same time instead of reducing the amount of the polyamideimide binder .

The above-mentioned binders adhere the electrode active material to the current collector surface and prevent the cathode from being short-circuited and peeled off by preventing the electrode active material from being separated from the surface of the current collector as the charge and discharge cycle of the battery progresses. Capacity and excellent cycle life characteristics. Particularly, the polyacrylic acid and the polyacrylic lithium have high electric conductivity compared to the same amount of other polymers and thus exhibit excellent high efficiency charge and discharge characteristics.

Therefore, the binder for the negative electrode material is not only uniformly applied to the electrode active material but also to the electrode current collector, as well as improving the binding force between the electrode binder and the current collector. Did you replace the binder suitable for high capacity anode materials as above? In the case of silicon, as the cycle progresses, the lithium metal substituted in the binder shows an effect of exceeding the lithium source in the electrochemical reaction, so that the cycle life and capacity can be improved. (See Table 1, 3)

The binder is preferably 6 to 16 weight percent (more preferably 8 to 13 weight percent) based on the total weight of the electrode binder, and the poly (alkylene oxide) polymer The acrylic mixture is preferably mixed in an amount of 0.1 to 100 weight percent (more preferably 1 to 50 weight percent) with respect to 100 weight parts of polyacrylic acid.

The anode active material used in the present invention may be silicon, silicon oxide (SiO _x , O <x <2), silicon and titanium (Ti), nickel (Ni), an alloy of iron (Fe) A composite of an oxide and a carbon material, a composite of a silicon alloy and a carbon; And at least one selected from the group consisting of these, but is not limited thereto.

The carbon material may be at least one selected from the group consisting of carbon, petroleum coke, activated carbon, carbon nanotubes, graphite and carbon fiber. The graphite may be mainly natural graphite or artificial graphite, But are not limited to.

The viscosity adjusting agent is a component which determines the binder density of the negative electrode and determines the bondability with the current collector, and may be added in an amount of 6 to 16 weight percent based on the total weight of the negative electrode mixture. Examples of the viscosity modifier include carboxyl methyl cellulose (CMC), carboxyethyl cellulose (CEC), ethyl cellulose (EC), hydroxymethyl cellulose (HMC), hydroxypropyl cellulose (HPC), carboxylethyl methyl cellulose CEMC). To control the viscosity of the water-soluble polymer, a solvent such as distilled water (DI water) and N-methyl pyrrolidone (NMP) is added to 100 parts by weight of the electrode slurry 10 To 35 weight percent of the composition may be used, but this must be done within 100 to 150 degrees Fahrenheit before or after polymerization or curing.

The conductive agent is a component for improving the conductivity of the negative electrode active material and may be added in an amount of 1 to 20 weight percent based on the total weight of the negative electrode mixture. Such a conductive agent is not particularly limited as long as it has electrical conductivity without causing a chemical change in the battery. Examples of the conductive agent that can be added include carbon black, carbon fiber, carbon nanotube, acetylene black, Ketjen Black, and superfine, but are not limited to those exemplified above.

The filler is an olefin-based polymer such as polyethylene or polypropylene as a component for suppressing the expansion of the negative electrode; Fibrous materials such as glass fiber, carbon fiber and the like can be used.

The current collector is not particularly limited as long as it is made of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foil, polymer coated with a conductive metal, and the like. A collector thickness of 3 to 500 is usually used.

Examples of the dispersion medium capable of dispersing the binder and the active material of the present invention include water, methanol, ethanol, propanol, isopropanol, butanol, isobutanol, s-butanol and t-butanol. can do.

Examples of the slurry solvent for electrodes include organic solvents such as NMP (N-methylpyrrolidone), DMF (dimethylformamide), acetone, dimethylacetamide, etc., or water. These solvents may be used alone or in combination of two or more.

The electrode active material slurry is prepared by dispersing water or an organic material into a dispersion medium and then adding a thickener, a dispersant, and a conductive material to the silicon negative electrode material, the silicon-carbon composite material and the carbon alloy negative electrode material, To prepare a slurry. In this case, the composition of the negative electrode differs according to each embodiment, and thus it is shown in the embodiment.

At this time, graphite powder having an average particle size of 18 micro (um) as shown in Fig. 9 was used as carbon of the active material used. The shape of the graphite may be amorphous, flat, flaky, or granular, and is not particularly limited. The graphite has an average particle size of 1 to 50 microns (占 퐉), preferably 5 to 30 microns (占 퐉).

The cathode composition ratio was 84: 1: 15 by weight of the active material, the conductive agent and the binder. In this case, the negative electrode active material used was silicon (average particle size, 20 to 30 nanometers), carbon black commercially available as a conductive material, and a binder synthesized in the present invention as a binder (Patent Application 10- 2012-0025118, March 12, 2012), and dissolved in distilled water (DI water) at 5 to 10 weight percent. Table 1 shows the comparison between the synthetic binder and the conventional binder. And polyacrylic lithium (PAA-Li) (substituted and modified form is shown in Fig. 1) in which the synthesized binder is modified by replacing the functional group with lithium in polyacrylic acid.

The electrode was prepared by mixing the anode active material and the conductive agent in a ball mill for 6 hours or more, adding a binder dissolved in 30 weight percent of a solvent, and mixing in a mixer for 30 minutes to prepare a slurry. Electrode coating on the current collector (copper) was carried out using Doctorblade to coat the previously prepared slurry with a constant thickness (50-150 um). At this time, the electrode was coated at 2.0 ~ 5.0 mg /, and the electrode was rolled to have a thickness of 20 ~ 50 microns after the preparation of the electrode, and the packing density was set at 1.2 ~ 1.6 g / cc. The coated sample was vacuum-dried at 80 to 110 degrees Celsius in consideration of the decomposition temperature of FIG.

The negative electrode for applying the slurry containing the binder and the electrode active material onto the current collector was manufactured according to a conventional method for evaluation in a manufacturer or an evaluation institution of a secondary battery.

The coated electrode was fabricated in a glove box under argon gas atmosphere. Before placing the electrode in the lower case, a drop of the electrolyte was dropped to prevent the electrode from moving, then the electrolyte was impregnated on the electrode, and the spreading membrane was raised. The gasket was raised, and lithium metal was punched and attached to a Sus-spacer. Then, a spacer was placed on the electrode, and the electrolyte solution was covered with the electrolyte. The assembled battery cell was stabilized by aging at room temperature for 12 hours and used for characterization.

Charge-Discharge Capacity and Life After 50 Cycles When Using Polyacrylic Lithium Binder Example Binder name Charging capacity
(mAh / g) Discharge capacity
(mAh / g) Initial efficiency
(%) 50 cycle life (%) Example 1 Polyacrylic lithium 3856
(4992) 3365
(4573) 87.2
(91.6) 70 Comparative Example 1
silicon Polyvinylidene fluoride 1080 926 85.7 10
(10 times) Comparative Example 2
silicon Polyacrylic acid 2138 1835 45.7 26 Comparative Example 3
silicon Polyvinyl alcohol 2530 1700 67.2 41

[Comparative Example 1]

As shown in Table 1, polyvinylidene fluoride was used as a binder instead of polyacyllithium, and the solvent was the same as in [Example 1], except that N-methylpyrrolidone was used instead of distilled water.

A silicon anode active material, a polyvinylidene fluoride binder, and a carbon black conductive agent were mixed at a weight ratio of 84: 1: 15 and dispersed in N-methylpyrrolidone solvent to prepare an anode slurry. The negative electrode active material slurry composition was applied to a current collector (copper) to prepare a negative electrode.

[Comparative Example 2]

A negative electrode was prepared in the same manner as in [Example 1], except that a negative electrode was prepared using polyacrylic acid as a binder, polyacrylic acid.

[Comparative Example 3]

A negative electrode was prepared in the same manner as in [Example 1], except that the negative electrode was prepared by changing the binder from polyacrylic lithium to polyvinyl alcohol (PVA).

Results of Example 1 < Tables 1 and 2 > As shown in FIGS. 2 and 3, it was confirmed that the initial charge capacity, initial efficiency and 50 cycle life were greatly improved by using a polyacrylic lithium binder. In particular, the use of a polyacrylic lithium binder resulted in a capacity retention rate (life span) of 70 percent after 50 cycles, which was much higher than that of other comparative binders. The reason for this is that as shown in FIG. 4 and FIG. 5, lithium and silicon from the charge and discharge processes react and alloy with each other. Also, the expansion rate was measured by a scanning electron microscope as shown in FIG. The expansion ratio (%) = [((electrode thickness after filling-electrode thickness before filling) / (electrode thickness before filling)] x 100

The negative electrode composition ratio was 88: 4: 8 by weight of a silicone-carbon active material: carbon black conductive agent: a binder in which 6: 2 carboxymethyl cellulose and low molecular weight polyacrylic acid were mixed. The electrode active material was synthesized by chemically treating silicon with carbon at an 8: 2 weight ratio. The silicon carbon composites synthesized by chemically treating silicon and carbon were those shown in Fig. The size of silicon used to chemically treat silicon and carbon is 10 To 50 nanometers (nm), preferably 20 to 30 nanometers (nm). The carbon shown in Fig. 10 was used. Average particle size is 1 to 50 microns (um), more preferably 5 to 30 microns (um).

The electrode manufacturing process is the same as in the first embodiment. The electrode was coated with 2.3 to 3.0 mg / liter and rolled to a thickness of 15 to 25 after the preparation of the electrode, and the packing density was set to 1.2 to 1.6 g / cc.

Electrochemical Properties of Binder Type for Silicon and Carbon Composite Active Materials Example 1 Binder name Charging capacity
(mAh / g) Discharge capacity
(mAh / g) Initial efficiency
(%) 50 cycle life
(%) Expansion ratio
(%) Example 2 Low molecular weight polyacrylic
Carboxymethyl Cellulose Rose 890 825 92 95
(20 times) 101 Comparative Example 1 Polyacrylic lithium 690 648 94 15
(10 times) 87 Comparative Example 2 Polyamideimide 840 795 95 63 108 Comparative Example 3 Polyacrylonitrile 887 827 93 60 69 Comparative Example 4 Carboxymethylcellulose 780 748 96 96
(20 times) 86 Comparative Example 5 Low molecular weight polyacrylic acid 880 821 93 89 80 Comparative Example 6 Polymer Polyacrylic acid 800 650 81 81
(35 times) 102

[Comparative Example 1]

The procedure of Example 2 was the same as that of Example 2 except that polyacrylic lithium was used in the case of mixing carboxymethylcellulose with a low molecular weight polyacrylic acid as a binder as in Example 2,

[Comparative Example 2]

The same as [Example 2] except that the binder of [Example 2] was replaced with a polyamideimide. The electrode manufacturing process is the same as in [Example 1].

[Comparative Example 3]

The same as [Example 2] except that the binder of [Example 2] was replaced with polyacryl. The electrode manufacturing process is the same as in [Example 1].

[Comparative Example 4]

The procedure of Example 2 was repeated except that the binder of [Example 2] was replaced by carboxymethyl cellulose. The electrode manufacturing process is the same as in [Example 1].

[Comparative Example 5]

The same as [Example 2] except that the binder of [Example 2] was replaced with a low molecular weight (molecular weight: 450,000) polyacrylic acid. The electrode manufacturing process is the same as in [Example 1].

[Comparative Example 6]

The same as [Example 2] except that the binder of [Example 2] was replaced with a high molecular weight (molecular weight: 1,250,000) polyacrylic acid. The electrode manufacturing process is the same as in [Example 1].

From the results of Example 2 and Comparative Examples 1 to 6, it can be deduced from Table 2 and FIG. 7 that the present invention is a result of preparing secondary battery characteristics by using a composite active material to which carbon is added to silicon. It has been found that the binder obtained by adding carboxymethyl cellulose to low molecular weight polyacrylic acid, carboxymethyl cellulose and low molecular weight polyacrylic acid maintains the cycle life without abruptly decreasing the lifetime. However, the expansion rate was slightly higher than other binders. However, considering the charging / discharging capacity, initial efficiency and expansion rate characteristics, the above-mentioned binder exhibited excellent characteristics when used. When the binder was used as the low molecular weight polyacrylic acid, the binding strength with the current collector was weak, but when the carboxymethyl cellulose was added to the low molecular weight polyacrylic acid, the binding power was good. If the cycle is performed for a long time, the silicone active material may be separated from the collector due to the volume expansion thereof, and the characteristics may be deteriorated rapidly. Therefore, in the composite system in which silicon is mixed with 20 weight percent of carbon, the most excellent binder has a low molecular weight The composite binder obtained by mixing carboxymethyl cellulose and polyacrylic acid in a weight ratio of 6: 2 was the best.

In order to increase the capacity of the negative electrode active material carbon, silicon is often added to the carbon. Therefore, in the present invention, it was confirmed that the composition of the active material of silicon, graphite, and carbon is suitable for lithium secondary battery characteristics when silicon is added to natural graphite and synthetic carbon and the mixing ratio is changed. As the binder, carboxymethyl cellulose and stadiene butadiene rubber binder were mixed at a weight ratio (6: 2) and used. At this time, the negative electrode was prepared with active material: conductive agent: binder = 88: 4: 8 at the negative electrode composition ratio. At this time, the ratio of the natural graphite to the silicon was 9: 1 by weight.

As shown in Table 3, when the amount of silicon was less than 10% by weight, simple mixing was used. In the case of the natural graphite used, the shape was roughly elliptical as shown in FIG. 10, Carbon (C) having a purity of 97%, an average particle size of 2 to 5 microns, and a specific surface area of 3.6 / g was used as the carbon material having an average particle size of 10 to 30 microns and a specific surface area of 5.4 / g. The electrode manufacturing process was the same as that of Example 1.

[Comparative Example 1]

The same as Example 3 except that the ratio of the active material to the ratio of the natural graphite: silicon: carbon = 9: 1: 0.05 weight percent was used as the active material in Example 3. The electrode manufacturing process was the same as in [Example 1].

[Comparative Example 2]

Example 3 is the same as Example 3 except that the ratio of the active material to the ratio of the natural graphite: silicon: carbon = 9: 1: 0.2 weight% of the natural graphite: silicon = 9: The electrode manufacturing process was the same as in [Example 1].

[Comparative Example 3]

Example 3 is the same as Example 3 except that the ratio of the active material to the ratio of the natural graphite: silicon: carbon = 8.55: 0.45: 1 weight percent was used as the active material in Example 3. The electrode manufacturing process was the same as in [Example 1].

[Comparative Example 4]

Example 3 is the same as Example 3 except that the ratio of the active material to the natural graphite: silicon = 9: 1 weight percent is changed to natural graphite: silicon: carbon = 8.1: 0.8: 1 weight percent. The electrode manufacturing process was the same as in [Example 1].

[Comparative Example 5]

In Example 3, the ratio of the active material to the active material was changed from natural graphite: silicon = 9: 1 weight percent to natural graphite: silicon: carbon = 9: 1: 1 weight percent, and the binder was mixed with carboxymethylcellulose, Was replaced with a polyamideimide in Example 3. The electrode manufacturing process was the same as in [Example 1].

[Comparative Example 6]

In Example 3, the ratio of the active material to the active material was changed from natural graphite: silicon = 9: 1 weight percent to natural graphite: silicon: carbon = 8: 2: 1 weight percent, and the binder was mixed with carboxymethylcellulose Was replaced with a polyamideimide in Example 3. The electrode manufacturing process was the same as in [Example 1].

[Comparative Example 7]

The battery fabrication process of Example 3 is the same, but the process of mixing natural graphite and silicon, which is an anode active material, is coated with pitch and then heat-treated in a nitrogen atmosphere of 900. Active material: Ketjen Black: And a binder (polyamideimide) = 8: 1: 1 (weight ratio). At this time, the mixing and coating process first has a size of 325 mesh, and the silicon is milled to 300 nanometers (nm) using a planetary mill and then mixed with natural graphite and pitch Respectively. At this time, the natural graphite was 10 percent by weight, and the fixed pitch amount was 5 percent of the total active material. In the mixing process, the pitch was dissolved in a tetrahydrofuran solvent, and then silicon and natural graphite were added thereto, and the solvent was evaporated by ultrasonic dispersion. The dried mixture was heat treated at 900 in a nitrogen atmosphere to obtain natural graphite coated with silicon particles and amorphous carbon. The silicon content in the above process was 10 weight percent.

The ratio of silicon to carbon composite active material and the initial efficiency and 50 cycle life division Active substance (total 88 weight percent) bar
sign
more Charge / discharge capacity (1C) Life after 50 cycles (%) Natural graphite silicon Carbon charge
(mAh / g) Discharge
(mAh / g) Initial efficiency (%) Example 3-1 9 One 0 Carboxymethylcellulose +
Stadiene butadiene rubber 405 400 98.8 35.0 Comparative Example
One 9 One 0.05 350 330 91.6 45.0 Comparative Example
2 9 One 0.2 310 293 94.5 40.0 Comparative Example
3 8.55 0.45 One 380 360 94.0 32.0 Comparative Example
4 8.1 0.8 One 300 295 96.0 20.0 Comparative Example
5 9 One One Polyamideimide 738 578 78 81 Comparative Example
6 8 2 One 684 522 76 48 Comparative Example
7 8 One One 785 536 68 28

The results of Example 3 show that when the binder is stadiene butadiene rubber and carboxymethylcellulose, the initial efficiency is higher when the silicone content is less than 10 weight percent, but the cycle life is 50 times lower. The reason for this is the effect of low purity carbon (97% purity) but the effect of simple mixing of silicon and natural graphite. However, when the polyamideimide binder was used and the silicone content was 10 weight percent, the lifetime after the 50 cycles was 81 percent. However, when polyamideimide is used as a binder in the case of using silicon as an active material, when the active material is coated with a pitch, the life cycle of 50 cycles when the amount of silicon is 20 wt% is drastically decreased.

In addition, as the silicon content was increased from 10 wt% to 30 wt%, the present carboxymethylcellulose stearate butadiene rubber binder or polyamideimide binder increased in initial capacity, but the capacity deterioration occurred due to a shortened life span.

Table 4 shows how the characteristics of the secondary battery are changed by varying the mixing ratio of the active material, the kind of the binder and the amount of the conductive agent in order to invent a suitable binder for the anode active material, i.e., the silicon alloy-carbon composite. Here, the active material has a ratio of a silicon alloy system containing titanium (Ti), nickel (Ni) and iron (Fe) of 34 weight percent to silicon of 66 weight percent and natural graphite at a ratio of 55:45, Meade and Ketjen Black as a conductive agent. The cathode composition ratio was the same as that of [Example 1] except that the silicon alloy-carbon composite: conductive agent: binder = 88: 4: 8 and the vacuum drying temperature of the electrode was less than 150.

[Comparative Example 1]

Example 4 is similar to Example 4 except that the electrode composition ratio was changed to silicon alloy-carbon composite: conductive agent: binder = 88: 4: 10 weight percent. The electrode manufacturing process is the same as in [Example 1] except that the vacuum drying temperature of the electrode is less than 150.

[Comparative Example 2]

Example 4 is similar to Example 4 except that the electrode composition ratio in Example 4 was changed to silicon alloy-carbon composite: conductive agent: binder = 90: 4: 6 weight percent. The electrode manufacturing process is the same as in [Example 1] except that the vacuum drying temperature of the electrode is less than 150.

[Comparative Example 3]

Example 4 is the same as Example 4 except that the electrode composition ratio in Example 4 was changed to silicon alloy-carbon composite: conductive agent: binder = 92: 4: 4 weight percent. The electrode manufacturing process is the same as in [Example 1] except that the vacuum drying temperature of the electrode is less than 150.

[Comparative Example 4]

Example 4 was the same as Example 4 except that the active material silicon alloy and carbon were changed to 8.9: 1.1 by weight ratio and the electrode composition ratio was changed to silicon alloy-carbon composite: conductive agent: binder = 90: 2: 8% by weight. The electrode manufacturing process is the same as in [Example 1] except that the vacuum drying temperature of the electrode is less than 150.

[Comparative Example 5]

Example 4 is similar to Example 4 except that the ratio of the active material silicon alloy: carbon composite is 8.9: 1.1 and the ratio of the electrode composition is silicon alloy-carbon composite: conductive agent: binder = 86: 4: 10 weight percent. The electrode manufacturing process is the same as in [Example 1] except that the vacuum drying temperature of the electrode is less than 150.

[Comparative Example 6]

Example 4 is the same as Example 4 except that the active material silicon alloy: carbon composite in Example 4 was used in a weight ratio of 9: 1 and the electrode composition ratio was changed to silicon alloy-carbon composite: conductive agent: binder = 90: 4: 6 weight percent. The electrode manufacturing process is the same as in [Example 1] except that the vacuum drying temperature of the electrode is less than 150.

[Comparative Example 7]

In Example 4, the binder was changed to a polyamide-imide polyvinyl alcohol and an active material silicon alloy: carbon = 8.9: 1.1 weight ratio, and the electrode composition ratio was changed to silicon alloy-carbon composite: conductive agent: binder = 90 : 2: 8 weight percent. &Lt; tb &gt; &lt; TABLE &gt; The electrode manufacturing process is the same as in [Example 1] except that the vacuum drying temperature of the electrode is less than 150.

[Comparative Example 8]

Carbon composite: conductive agent: binder = 90 (weight ratio) was used in Example 4, and the ratio of the active material silicone alloy: carbon = 8.9: 1.1 weight ratio and the binder was carboxymethyl cellulose + stadiene butadiene rubber : 2: 8 weight percent. &Lt; tb &gt; &lt; TABLE &gt; The electrode manufacturing process is the same as in [Example 1] except that the vacuum drying temperature of the electrode is less than 150.

[Comparative Example 9]

The ratio of the active material silicone alloy: carbon composite = 8.9: 1.1 by weight in Example 4 and the weight ratio of hydroxypropylcellulose to polyamideimide was 5.6: 2.4, and the electrode composition ratio was changed to silicon alloy-carbon composite: 88: 4: 8 weight percent. The electrode manufacturing process is the same as in [Example 1] except that the vacuum drying temperature of the electrode is less than 150.

The results of Example 4 show that when the binder is polyamideimide, when the silicone alloy content is 55 weight percent, the initial efficiency and the 50 cycle life are slightly higher, but the initial capacity is lower. The reason for this is considered to be the capacity below the actual theoretical capacity due to the low silicon content. On the other hand, when the amount of binder is 4% by weight, the life span is lowered due to lack of adhesion with the current collector.

When the amount of the silicon alloy in the negative electrode active material is 89 weight percent and the remainder is graphite, when the negative electrode composition is in the range of 86 to 90: 2 to 4: 6 to 10 in the range of the active material: conductive agent: binder, Considering the initial capacity, the binder was polyamideimide and the electrode composition was 86: 4: 10. A binder composed of 89 weight percent silicon alloy and 11 weight percent natural graphite in which initial charge capacity and expansion rate are reduced is a binder to which polyamideimide is added with 5.6: 2.4 by weight of hydroxypropyl cellulose.

Initial Efficiency, Expansion Rate, and Cycle Life Characteristics of Silicon Alloy-Carbon Composite System with Active Material and Binder sample Active material Conductive agent
(KB) bookbinder Charging capacity (mAh / g) Discharge capacity (mAh / g) Initial efficiency (%) Expansion ratio
(%) 50 cycle life (%) Silicon-alloy Example 4 88
(Silicon-alloy: natural graphite =
55.45) 4 8 (polyamideimide) 749 622 83 140 85.3 Comparative Example 1 88
(Silicon-alloy: natural graphite =
55.45) 4 10 (polyamideimide) 877 702 80 - 72 Comparative Example 2 90
(Silicon-alloy: natural graphite =
55.45) 4 6 (polyamideimide) 809 711 87.8 - 60 Comparative Example 3 92
(Silicon-alloy: natural graphite =
55.45) 4 4 (polyamideimide) 542 415 76.5 85 44 Comparative Example 4 90
(Silicon-alloy: natural graphite =
8.9: 1.1) 2 8 (polyamideimide) 1063 911 85.6 94 80
(25 times) Comparative Example 5 86 (silicon alloy: natural graphite 8.9: 1.1) 4 10 (polyamideimide) 1053 879 83.2 120 92.9
(25 times) Comparative Example 6 90
(Silicon-alloy: natural graphite =
9: 1) 4 6 (polyamideimide) 949 787 82.9 104 70
(25 times) Comparative Example 7 90
(Silicon-alloy: natural graphite =
8.9: 1.1) 2 8 (PVA) 944 733 77.6 106  - Comparative Example 8 90
(Silicon-alloy: natural graphite =
8.9: 1.1) 2 8 (carboxymethyl cellulose + stadiene butadiene rubber) 1066  859 80.5 140 - Comparative Example 9 88
(Silicon-alloy: natural graphite =
8.9: 1.1) 4 8 (polyamideimide: 5.6, hydroxypropylcellulose: 2.4) 912 674 73.9 81.8 -

Claims (3)

The binder to be added to the composite secondary battery negative electrode active material mixed with 80wt% of silicon and 20wt% of carbon is composed of polyacrylic acid and carboxymethyl cellulose having an aqueous low molecular weight of 450,000, and the weight ratio of the polyacrylic acid and carboxymethyl cellulose is 6: Wherein the negative electrode is a negative electrode.
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